This invention relates to a wire cut type electric discharge machine which can start an electric discharge machining operation even if its wire electrode is in contact with a workpiece at a machining start position.
FIG. 1 shows the arrangement of a conventional wire cut type electric discharge machine. In FIG. 1, reference numeral 1 designates a workpiece to be machined; 2, an X-Y table on which the workpiece 1 is mounted, the X-Y table being moved two-dimensionally; 3, a jig for fixing the workpiece 1 to the table 2; 4, a wire electrode; 5, a machining power source for applying a machining voltage across the workpiece 1 and the wire electrode 4; 6, an X-axis motor for moving the table 2 in an X-axis direction; 7, a Y-axis motor for moving the table 2 in a Y-axis direction; 8, nozzles for applying a machining solution 9 to a machining gap formed between the workpiece 1 and the wire electrode 4; 10, an NC (numerical control) device for controlling the operations of the X-axis motor 6 and the Y-axis motor 7 and the operation of the power source 5; 11, a machining solution supplying device for supplying the machining solution through the nozzles 8 to the machining gap; 12, a voltage detecting device for detecting the voltage across the workpiece 1 and the wire electrode 4 (hereinafter referred to as "an interelectrode voltage" , when applicable); and 49, a perforated paper tape containing a machining numerical control program.
FIG. 2 shows the arrangement of the NC device 10 in detail. In FIG. 2, reference numeral 41 designates a CRT (cathode ray tube) on which operating conditions of the wire cut type electric discharge machine are displayed; 42, an operating switch board for operating the NC device 10; 43, a machining solution control device for controlling the operation of the machining solution supplying device 11; 44, a machining power source controlling device for controlling the machining power source 5; 45, a motor controlling device for controlling the X-axis motor and the Y- axis motor; 46, a machining voltage evaluating device for evaluating the voltage detected by the voltage detecting device 12, to determine whether or not the wire electrode is in contact with the workpiece; 47, a backward movement controlling device which, when the wire electrode is brought into contact with the workpiece, moves the wire electrode backwardly along the machining locus to leave from the workpiece to start the machining operation again; 49, a perforated paper tape containing an NC program; and 48, an NC program interpreting device for interpreting an NC program (machining program) given by the paper tape 49 and a memory (not shown).
FIG. 3 is a diagram showing the positional relationship between the wire electrode and the workpiece. In FIG. 3, reference numeral 60 designates a machining start hole formed in the workpiece; 63, a machining start point where the wire electrode 4 is positioned at the start of a machining operation; and 8, the nozzles described hereinabove with reference to FIG. 4.
The operation of the electric discharge machine thus constructed will be described. A product is formed by machining the workpiece 1 as follows:
The workpiece 1 is fixedly mounted on the X-Y table 2 and the wire electrode 4 is inserted into the machining start hole 60 formed in the workpiece 1. Then, the paper tape 49 is set in the NC program interpreting device 48. Upon depression of the machining start button, the NC program interpreting device 48 starts interpreting the NC program to provide instructions to carry out a machining operation.
The machining operation is performed by using the discharge energy which is produced when a voltage is applied across the wire electrode 4 and the workpiece 1. Hence, the wire electrode 4 must be spaced from the workpiece 1 to the extent that electric discharge takes place therebetween. Even if, in the case where the wire electrode 4 is in contact with the workpiece 1, voltage is applied across them, no electric discharge occurs; that is, the machining operation is not advanced. When the wire electrode is brought into contact with the workpiece, the application of a movement instruction by the NC program interpreting device 48 is suspended, and instead the backward movement controlling device 47 supplies a movement instruction so as to move the wire electrode backwardly along the machining locus. As a result, the wire electrode 4 and the workpiece 1 are moved away from each other to remove the contact to thereby start the electric discharge again. Then, the wire electrode is returned to the position from which it has been moved backwardly. Thus, the operation of the backward movement controlling device 47 is ended. Under this condition, the machining operation is carried out again in response to a movement instruction from the NC program interpreting device 48.
The conventional wire cut type electric discharge machine thus constructed suffers from the following difficulties in forming a product by machining a workpiece: In starting a machining operation, the wire electrode 4 is inserted into the machining start hole 60. In this operation, the position of the wire electrode 4 determined by the machining program; that is, the machining start position 63 may be shifted from the machining start hole 60 as shown in FIG. 6. If, in this case, the wire electrode 4 is in contact with the workpiece 1, the machining operation will not be carried out, because the program has no locus along which the wire electrode 4 is moved backwardly to remove the contact with the workpiece, and therefore the wire electrode 4 and the workpiece 1 are maintained in contact with each other. That is, in this case, the operator must manually separate the wire electrode and the workpiece from each other, to start the machining operation.